The present invention relates to instruments and processes for analyzing aerosol particles, and more particularly to apparatus for characterizing the particles as to their size and the presence of biological components.
Aerosols composed of biological particles comprise a small fraction of the aerosols present in our atmosphere. Nonetheless, there is an increasing interest in analyzing biological aerosols, which can incorporate bacteria, fungi and pollens. Certain diseases, for example, tuberculosis, influenza and pneumonia, are transmitted via airborne particles or droplets. Diseases that affect livestock and other farm animals, (e.g. anthrax and brucellosis) and diseases that affect crops, likewise are transmitted through the air. Airborne pollens cause allergic reactions in humans. The Persian Gulf War, and recent news reports about individuals in possession of anthrax, have galvanized public concerns over biological warfare and the potential for terroristic release of airborne biological contaminants. The study of airborne fungi, pollen and other components is now recognized as a key concern in the control of indoor environments, e.g. office buildings. Thus, it is not surprising to see the study of biological aerosols playing an increasing role in such diverse areas as epidemiology and other medical fields, agriculture, building management and defense.
A variety of devices have been used to detect and identify airborne biological particles, e.g. pollen traps, impingers, impactors, and cyclones that collect particles for subsequent identification. These methods fail to deliver real time results, and are time consuming and labor intensive, particularly in cases that require growing bacteria.
Similarly, a variety of particle sizing methods and systems are known. For example, U.S. Pat. No. 5,561,515 (Hairston, et al), assigned to the assignee of the present application, discloses an instrument for measuring the aerodynamic size of particles in the micron and submicron ranges. Particles in series are accelerated through a nozzle, and upon exiting the nozzle encounter two relatively long wavelength (red or infrared) laser beams. Aerodynamic size is determined based on a time-of-flight measurement, i.e. the transit time of each particle from the first beam to the next. Transit time is measured by detecting light scattered by the particle as it travels through both beams. A partial overlapping of the beams is used advantageously to reject any erroneous readings from single-trigger or coincidence events.
It has been found that biological cells contain fluorescent molecules, e.g. flavins, amino acids and nicotinamide adenine nucleotides, and thus emit fluorescent signals when exposed to excitation energy within a range of excitation frequencies. With this in mind, U.S. Pat. No. 5,701,012 (Ho) discloses a device for determining aerodynamic sizes, and subjecting the particles in sequence to ultraviolet laser light, then detecting any fluorescence. The ultraviolet laser is operated in the CW (continuous wave) mode, so that excitation occurs as particles reach that part of the particle path intersected by the UV laser. A similar technique is discussed in an article by Pinnick et al entitled "Fluorescence Particle Counter for Detecting Airborne Bacteria and Other Biological Particles", Aerosol Science and Technology, Volume 23, pp. 653-664 (1995). The continuous application of laser energy avoids the need to time individual particles for timing a pulsed UV laser. At the same time, it is difficult to maintain continuous lasers at the high power levels necessary to achieve the particle irradiation energy needed to trigger readily detectable fluorescent emissions. Moreover, continuous lasers maintained at any power are inefficient in this application, given that the system requires only intermittent irradiation.
Another approach, disclosed in U.S. Pat. No. 5,681,752 (Prather), involves determining aerodynamic size and chemical composition of particles. A stream of particles is drawn through a vacuum device with several stages separated by skimmers. Each particle is accelerated to a speed inversely related to its aerodynamic size, then remains substantially at that speed as it moves through two spaced apart beams. The travel time from one beam to the next yields a timing signal used to fire a high intensity laser downstream of the velocity measuring beams. The high intensity laser desorbs the particles and ionizes their molecule. The ions are then subjected to mass spectrometry.
Although measuring the particles at constant velocities affords relative simplicity in timing the downstream laser, the constant velocities are attainable only in a near vacuum (typically 10.sup.-6 to 10.sup.-7 torr), which requires high powered vacuum pump and inlet skimmers. A high intensity laser is required to free molecular ions from the particles. The mass spectrometry stage requires significant additional hardware for post irradiation analysis, including electrically charged ion accelerating plates, a flight tube for ion separation, and an ion detector. The travel of ions through the flight tube takes on the order of hundreds of microseconds, and thus limits the rate at which particles can be measured.
Therefore, it is an object of the present invention to provide a system capable of generating real time information about particle size and composition, using a relatively low power, pulsed laser or other source of excitation energy.
Another object is to provide an aerosol analysis system in which time-of-flight measurements, taken individually during particle accelerations are rapidly converted into timing information used to intermittently generate excitation energy.
A further object is to provide, in a system for generating aerodynamic size information based on scattered light and particle composition information based on induced particle emissions, a detecting arrangement with improved sensitivity to scattered light and emitted signals.
Yet another object is to provide a process that combines relatively low energy lasers for time-of-flight measurements and excitation of particles with fluorescent components, with enhanced sensitivity for detecting scattered and emitted energy, to generate more reliable real time particle size and composition information.